Photoelectric scanning apparatus

ABSTRACT

A photoelectric scanning apparatus which detects one or more rows of information encoded in or on travelling webs or interconnected photographic films or prints has one or more pairs of mirror symmetrical light-conducting rods which are embedded in a support adjacent to the path of the web so that their axes intersect at a point of the path which is in line with the row of information to be scanned. The support is formed with two chambers each of which is located behind a different rod and each of which receives a removable clip. One of the clips contains one or more removable lamps and the other clip contains one or more removable photosensitive elements. A lamp in the properly inserted clip emits a light beam which passes through the respective rod and is normally reflected on the travelling web to thereupon pass through the other rod and to impinge on the photosensitive element behind the other rod. If the encoded information is in the form of notches or perforations, the reflection of light is interrupted whenever a notch or perforation register with the intersection of the axes of the rods whereby the photosensitive element generates a signal.

United States Patent Hujer et a].

May 8, 1973 [54] PHOTOELECTRIC SCANNING APPARATUS [75] Inventors:Friedrich Hujer, Grunwald; Harald Fengler, Munich, both of Germany [73]Assignee: Agl'a-Gevaert Aktiengesellschaft,

Leverkusen, Germany [22] Filed: Sept. 29, 1971 [21] Appl. No.: 184,797

[30] Foreign Application Priority Data Oct. 2, 1970 Germany ..P 20 48414.4

[52] US. Cl ..250/219 FR, 250/227 [51] Int. Cl. ..G01n 21/30 [58] Fieldof Search ..250/219 FR, 219 DC,

[56] References Cited UNITED STATES PATENTS 3,278,754 10/1966 Wallace..250/219 DC 1,969,465 8/1934 Jones ..250/219 FR X 3,308,301 3/1967Weissbach... ..250/219 FR 3,533,657 lO/1970 Da Silva ..250/219 DC3,573,478, 4/1971 Stempler ..250/239 Primary ExaminerWalter StolweinAttorney-Michael S. Striker [57] ABSTRACT A photoelectric scanningapparatus which detects one or more rows of information encoded in or ontravelling webs or interconnected photographic films or prints has oneor more pairs of mirror symmetrical light-conducting rods which areembedded in a support adjacent to the path of the web so that their axesintersect at a point of the path which is in line with the row ofinformation to be scanned. The support is formed with two chambers eachof which is located behind a different rod and each of which receives aremovable clip. One of the clips contains one or more removable lampsand the other clip contains one or more removable photosensitiveelements. A lamp in the properly inserted clip emits a light beam whichpasses through the respective rod and is normally reflected on thetravelling web to thereupon pass through the other rod and to impinge onthe photosensitive element behind the other rod. If the encodedinformation is in the form of notches or perforations, the reflection oflight is interrupted whenever a notch or perforation register with theintersection of the axes of the rods whereby the photosensitive elementgenerates a signal.

8 Claims, 3 Drawing Figures PI-IOTOELECTRIC SCANNING APPARATUSBACKGROUND OF THE INVENTION The present invention relates to scanningapparatus in general, and more particularly to improvements inphotoelectric scanning apparatus. Still more particularly, the inventionrelates to improvements in photoelectric scanning apparatus which can beused with advantage to detect information encoded on travelling bodies,such as webs of paper, photographic film or the like, or to producesignals in response to detection of brighter or darker spots or voids onor in the travelling bodies.

A drawback of presently known photoelectric scanning apparatus which areused for detection of information encoded on or otherwise present intravelling bodies is that all component parts of the scanning apparatusmust be produced and mounted with a high degree of precision. Thus, therelative positions of the light source and the photosensitive transducerwhich is exposed to light emitted by the light source and normallyreflected on the travelling body must be selected with a very highdegree of accuracy. This presents serious problems whenever it becomesnecessary to inspect, clean and/or replace the light source and/or thetransducer. Thus, the dimensions of spare light sources and/ortransducers must match exactly the dimensions of the light source andtransducer which are in actual use, and the means for mounting suchparts adjacent to the path for a travelling body must be designed withutmost precision to insure proper reflection of light on a predeterminedarea of the body, namely, on the area which carries a row of encodeddata. Therefore, many photoelectric scanning apparatus are of theexpendable type without any possibility of replacing a defective lightsource and/or transducer.

SUMMARY OF THE INVENTION An object of the invention is to provide anovel and improved photoelectric scanning apparatus wherein all partswhich are likely to require inspection, cleaning and/or replacement arereadily accessible and wherein minor or major differences between thedimensions of several removable component parts do not affect theaccuracy of the scanning action.

Another object of the invention is to provide a photoelectric scanningapparatus which is particularly suited to scan information which isencoded on travelling webs of paper, photographic film or the like andwhich is capable of discriminating between two or more different typesof information.

A further object of the invention is to provide a photoelectric scanningapparatus which can be installed in or on many existing machines, suchas developing, printing, copying and packing machines for photographicfilm and/or photographic prints, to serve as a superior substitute forpresently known scanning apparatus.

An additional object of the invention is to provide a scanning apparatuswherein one or more light sources and/or one or more transducers can bereached, in-

spected, cleaned and/or replaced by semiskilled or unskilled persons andwhich insures optimal guidance of travelling material at the scanningstation or stations.

Still another object of the invention is to provide a photoelectricscanning apparatus which, though eminently suited for detection ofinformation which is encoded in the form of opaque or bright symbols, isparticularly suited for detection of perforations, holes, notches orlike voids in a travelling sheet-like body.

The invention is embodied in an apparatus for photoelectrically scanningwebs of flexible material, such as photographic films or paper strips,which are transported along a predetermined path. The apparatuscomprises support means which is adjacent to one side of the path for aweb, at least one pair of substantially rod-shaped mirror symmetricallight-conducting members which are mounted in the support means in suchpositions that the extensions of their axes intersect each other at apredetermined point of the path for the web, namely, at a point which isin line with a row of encoded data provided on the web and beingintended to be scanned by the apparatus, a light source position edbehind one of the light-conducting members so that a beam of light whichissues from such source and passes through the one member is normallyreflected by a web in the path and impinges against the other member topass lengthwise through such other member, and a photosensitive elementpositioned behind the other member to be exposed to reflected light. Theabsence of reflected light or the presence of such light can cause theelement to produce signals which .are thereupon" amplified (ifnecessary) and used to control the transport, severing and/or othermanipulation of the web.

The light-conducting members are preferably provided with smooth endsurfaces which face the aforementioned'point in the path for the web andextend at vantages thereof, will be best understood upon perusal of thefollowing detailed description of certain specific embodiments withreference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a partly side elevational andpartly sectional view of a photoelectric scanning apparatus whichembodies the invention;

- FIG. 2 is a transverse vertical sectional view asseen' in thedirection of arrows from the line II-II of FIG. 1;

and

FIG. 3' is a fragmentary partial plan view and partial horizontalsectional view of an insert which is utilized in the scanning apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS.

Referring first to FIG. 1, there is shown a frame or housing 101 whichsupports two rotary pressing rollers 3, 4, a pair of cooperatingadvancing rolls or wheels 6 7, and a block-shaped support 1. The latterhas a convex guide surface 1a which is adjacent to and faces theundersides of the pressing rollers 3, 4. These rollers define with theguide surface la an elongated path for lengthwise transport of a web 2which can constitute a series of spliced-together photographic customerfilms or a strip of paperconsisting of a succession of photographicprints. If the web 2 consists of a series of films, such films can bespliced together by uniting bands prior to introduction into acontinuous developing machine. It is customary to provide such filmswith encoded information in the form of perforations or notches whichare applied in the region of splices between successive films to allowfor automatic detection of splices for the purpose of subdividing theweb into discrete films which'are thereupon returned to customers.Furthermore, such web is normally provided with additional encodedinformation, such as notches, perforations or graphite marks, one foreach film frame, to facilitate accurate positioning of film frames in aphotographic printing or copying machine. Such encoded information isused in many photographic processing laboratories bedause it allows forrationalization of the developing, printing and sorting operation. Asmentioned above, the information which is encoded in the region ofsplices is scanned subsequent to completion of the developing (and, ifnecessary, printing or copying) operation to allow for automaticdetection of splices for the purpose of arresting the web at necessaryintervals so as to separate the films from each other and to allow-formanual or automatic insertion of separated films into customercontainers or envelopes. The information which is encoded in the regionof each film frame is scanned in a printing or copying machine to allowfor automatic stoppage of the web whenever a fresh film frame registerswith the printing or copying station.

If the web 2 consists of a series of photographic prints which areobtained by reproducing theimages of film frames onto a photosensitivepaper, the web is provided with encoded information in the region ofeach print so as to allow for automatic scanning of such informationand, intermittent stoppage of the web for the purpose of severingsuccessive prints from each other,

, for example, in order to allow for manual accumulation with a severingdevice, or to place the frame lines between successive photographicprints into register with a severing device,

The curvature of the convex guide surface la on the support 1 preferablycorresponds to that of an are which forms part of a circle. It ispreferred to provide the surfacela with one or more cutouts, especiallyif the web 2 consists of a series of films which are spliced to eachother end-to-end, so that the surfaces at the undersides of the filmsare contacted only by two transversely spaced relatively narrow ribs lb(see particularly FIG. 2) each of which contacts the respective marginalportion of the adjacent film. The scanning station wherethe web 2 isscanned for the presence of absence of encoded information in the formof perforations, marginal notches, graphite marks or symbols which areexposed onto the photosensitive layers of films is located midwaybetween the pressing rollers 3 and 4. Each of these rollers can bebiased toward the adjacent portion of the guide surface la by one ormore springs (not shown) or is mounted in such a way that it can rest onthe adjacent portion of the web 2 by gravity. The direction in which theweb 2 is transported lengthwise by the advancing rolls 6, 7 is indicatedby an arrow 5. At least one of the advancing rolls 6, 7 is driven in amanner not forming part of the present invention and preferably in sucha way that the drive can be arrested in response to signals produced bythe improved photoelectric scanning apparatus on detection of encodedinformation. The pressing rollers 3, 4 produce a desirable brakingaction by biasing the-adjacent portions of the travelling web 2 againstthe stationary guide surface 1a to thus insure that the web follows theoutline of the surface 1a and remains in its predetermined path duringtravel past the scanning station between the pressing rollers.

The guide surface 1a of the fixedly mounted support 1 is provided withthree transversely spaced continuous uninterrupted grooves 1c (seeparticularly FIG. 2) each of which extends all the way between the twovertical edge faces of the support. The spacing of the grooves 1c fromeach other corresponds to the spacing of different types of encodedinformation on the travelling web 2. For example, a row of marginalnotches in the web 2 can pass above the leftmost groove 10 of FIG. 2; arow of perforations may pass along the median groove. 10 of FIG. 2; anda second row of marginal notches can pass above the right-hand groovelc'of FIG. 2. One row of notches can denote successive film frames, theother row of notches can denote the splices between successive films ofthe web 2, and the perforations can denote customer data, such asaccount numbers, customer addresses, the number of prints desired,and/or other information. The improved scanning apparatus comprisesthree transverselyspaced photoelec-. tric detectors the first of whichcan detect the notches in one marginal portion, the second of which candetect the notches in the other marginal portion and the third of whichcan detect the perforations in the intermediate portion of the web 2.However, it is equally' possible to provide the improved scanningapparatus with a single photoelectric detector or with two,,four or moredetectors, depending on the number of different data which are appliedto the web. 7

Each detector of the scanning apparatus comprises two mirror symmetricallight-conducting members in the form of bars or rods 8 and 9. The frontor upper end portions of each pair of light-conducting members(hereinafter called rods for short) extend beyond the bottom surface ofthe respective groove 1c toward but short of the adjacent point P of thepath for the web 2-. The support 1 is provided with prefabricated boresle for reception of the rods 8, 9 and each such rod can be bonded to thesupport 1, for example, by resorting to a suitable adhesive. These rodsmay consist of vitreous or synthetic plastic material and the extensionsof the axes of each pair of associated rods 8, 9 intersect'each other atthe predetermined point P of the path for the web 2, namely, at thatpoint which is in register with the row'of notches, perforations orother encoded information which is to be scanned by the respectivedetector. In the illustrated embodiment, the axes of the rods 8, 9 ofeach pair of associated rods make acute angles with a symmetry plane2020 which is located midway between the rods 8, 9 and includes thepoint P in the path for the web 2 where the axes of such rods intersecteach other. The front or upper end surfaces 8a, 9a of the rods 8, 9shown in FIG. 1 are normal to the axes of the respective rods and facethe point P where such axes intersect each other in the path for the web2. The surfaces 8a, 9a are preferably smooth by being ground, polishedor otherwise treated to a high degree of precision finish. The distancebetween the front end surfaces 8a, 9a and the point P where the axes ofthe rods 8, 9 shown in FIG. 1 intersect each other preferablyapproximates or equals the diameter of a rod 8 or 9. The width of eachgroove 10 in the support 1 preferably at least equals but normallyslightly exceeds the diameter of a rod 8 or 9.

The support 1 is further provided with two substantially slot-shapedchambers 1d and 1d which are respectively located behind the rear endsurfaces of the rods 8 and 9. The chambers 1d, 1d are machined into thesupport 1 and extend at right angles to the plane of FIG. 1. The chamber1d accommodates a removable insert or clip 10 which contains a batteryof several (preferably three) light sources 12 in the form of electriclamps. The chamber 1d accommodates a similarly dimensioned block-shapedinsert or clip 11 which contains several (preferably three)photosensitive elements or transducers 112 (one shown in FIG. 1). Theinsert 10 is formed with parallel bores 10a each of which receives oneof the light sources 12 and the insert 11 is provided with similar bores(not specifically shown) for the photosensitive elements 112. As shownin FIG. 3, each light source 12 includes a transparent envelopesurrounding a filament l2a-and having a front end portion 12b whichconstitutes a lens to focus the light beam issuing from the respectivefilament 12a upon the rear end surface of the respective rod 8 in thesupport 1. The rear or lower end portions of the light sources 12 havereduced diameters and are received in complementary portions of therespective bores 10a in the insert 10. The latter contains at least asmany light sources 12 as there are rods 8 in the support 1, and the lens12b of each light source is located behind the rear end surface of adifferent rod 8 when the insert 10 is properly accommodated in thechamber 1d of the support 1.

The dimensions of each photosensitive element or transducer 112 in theinsert 11 preferably equal or approach the dimensions of a light source12. Such transducers may also comprise transparent envelopes of glass orsynthetic plastic material having lenticular upper end portions servingto focus reflected light onto a suitable photosensitive surface in theenvelope. The narrower or smaller-diameter lower end portions of thelight sources 12 and transducers 112 are just large enough to permit forproper introduction of suitable conductors which connect the lightsources 12 with a source of electrical energy, not shown, and thetransducers 112 with the device which receives signals from therespective detector (e.g., a severing device or a device which drivesthe advancing rolls 6, 7). The inserts 10, 11 preferably (but notnecessarily) resemble bricks and may be held in the respective chambers1d, 1d by friction or by suitable detent means, not shown. In theillustrated embodiment, the inserts 10, 11 are respectively held inoperative positions by tightening and retaining screws 13, 14 shown inFIG. 1.

The operation:

The user threads the leader of a web 2 between the guide surface 1a ofthe support 1 and the pressing rollers 3, 4 and thereupon through thenip of the advancing rolls 6, 7. When the drive for the advancing roll 6and/or 7 is started, the web 2 moves lengthwise in the directionindicated by the arrow 5 and follows closely the outline of the guidesurface 1a so that its notches, perforations, graphite marks and/orother indicia can be properly scanned by the light beams issuing fromthe filaments 12a of light sources 12 in the insert 10. It is assumedthat the light beam issuing from the source 12 behind the rod 8 of FIG.1 impinges upon a row of perforations in the web 2. Such light beamimpinges upon the web 2 at the point P and at an angle of 45 to thesymmetry plane 2020 and is reflected at the same angle to pass throughthe rod 9 of FIG. 1 and to impinge upon the corresponding transducer112. The curvature of the guide surface 1a on the support 1, combinedwith the biasing action of the pressing rollers 3, 4, prevents anytransverse curling of the web 2 at the scanning station (point P), i.e.,the web remains in its predetermined path; consequently, the beam oflight issuing from the rod 8 at the front end surface 8a invariablyfinds an oncoming perforation and passes therethrough so that noreflection takes place and the corresponding transducer 112 produces asignal which is transmitted to the drive for the advancing roll 6 and/or7 or to another device which receives signals from the detectorincluding the rods 8, 9 of FIG. 1. If the web 2 consists of a series ofspliced-together exposed and developed photographic films, thereflectivity of such films is highly satisfactory so that the differencebetween the amounts of light which reach the transducer 112 when a lightbeam is reflected on the web 2 and the amounts of light which reach thetransducer 112 when the light beam issuing from the rod 8 can passthrough a perforation at the point P is very pronouncedjThe lens at theupper or forward end of the envelope of the transducer 112 behindthe rod9 of FIG. 1 focusses reflected light on the photosensitive surface whoseresistance to the flow of electric current is very low in response tostrong illumination and very high in the absence of illumination. Thelens 12b on the envelope of the light source 12 behind the rod 8 of FIG.1 focusses the light beam on the rear end surface of the rod 8. The factthat some stray light might reach the transducer 112 behind the rod 9when the light beam issuing from the rod 8 and passing through aperforation at the point P cannot be reflected against the surface 9a ofthe rod 9 does not affect the accuracy of measurement because the amountof stray light is very small, particularly when compared with the amountof light which reaches the rod 9 when a perforation has moved beyond thepoint P. The signal produced by the transducer 1 12 can be readilyamplified, shaped and/or otherwise processed to serve for regulation ofthe operation of one or more devices, such as the drive for the roll 6and/or 7, a severing device for the web 2 or a counter of signals. Thecounter can be used to arrest the drive for the roll 6 and/or 7 inresponse to detection of a predetermined number of perforations, forexample, if each film of the web 2 is to be subdivided into sections ofpredetermined length.

Dust or other foreign matter which is entrained by the web 2 and happensto descend onto the highly polished front end surfaces 811, 9a of therods 8, 9 shown in FIG. 1 descends by gravity into the respective grooveof the support 1. If the groove 1c is filled with dust, the additionallyadmitted dust is carried away by the underside of the web 2 withoutaffecting the accuracy of the scanning action.

It was found that the positioning of rods 8, 9 in a manner as shown inFIG. 1 (at an angle of about 45 with reference to the symmetry plane-20) is a satisfactory compromise between a compact design and anoptimum angle of reflection. The amount of reflected light isparticularly high if the angles between 'the plane 20-20 and the axes ofthe rods 8, 9 are small.

An important advantage of the improved photoelectric scanning apparatusis that minor or even major deviations of the dimensions of lightsources 12 and/or transducers 112 from a standard size do notappreciably affect the accuracy of the scanning action. This isattributed to the provision of light-conducting rods 8 9 which arefirmly embedded in the support 1 so that their positions with referenceto the path for the web 2 remain unchanged. Therefore, the exactpositioning of lightsources 12 and transducers 112 behind the rearendsurfaces of the rods 8 and 9 is of lesser importance. It suffices ifthe dimensions of all light sources 12 approximate each other and if alight source 12 which is inserted in place of-a damaged light sourceassumes in the insert 10 a position which is at least close to theposition of the previously employed light source. The same holds truefor the transducers 112. It was found that even substantial differencesbetween the distance from the rear end surface of a rod 8 to thefilament "12a of a removed light source 12 and the distance from therear end surface of such rod 8 and the filament 12a of a freshlyinserted light source 12 cannot appreciably affect the'light guidingaction of the rod 8 so that the accuracy of. the scanning action remainspractically unchanged. This is of advantage because the replacement of aspent light source 12 or transducer 112 can be carried out by anunskilled attendant and the apparatus will continue to operateproperlyeven if the newly inserted part 12 and/or 112 is not in an optimumposition with reference to the respective light-conducting rod.

As mentioned above, the improved scanning apparatus is susceptibleofmany further modifications and can be used in connection with thescanning of a wide variety of travelling bodies. For example, thephotosensitive element or elements can be designed to produce signals inresponse to reflection of light on opaque spots of a travelling paperweb or the like, in response to detection of data which are exposed ontoa web consisting of photographic films, or in response to detection ofspecially configurated perforations or notches. Furthermore, onedetector of the scanning apparatus may be designed to produce signals inresponse to detection of opaque data and the other detector or detectorsmay be designed to produce signals in response to detection of brightspots, or vice versa.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omittingfeatures whichfairly constitute essential characteristics of the generic and specificaspects of our contribution to the art and, therefore, such adaptationsshould and are intended to be comprehended within the meaning and rangeof equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

We claim:

1. Apparatus for photoelectrically scanning webs of flexible material,such as photographic films or paper webs, particularly to detectportions of reduced reflectivity, comprising support means having aconvex guide surface defining a portion of an elongated path for webs offlexible material; means for advancing successive webs along said pathin a predetermined direction; means for biasing the webs in said portionof said path against said guide surface; a light source removablymounted in said support means; a photosensitive element removablymounted in said support means; a first rod-shaped light-conductingmember at least partially embedded in said support means between saidlight source and said guide surface to direct a beam of light whichissues from said light source against the web at a predetermined pointof said portion of said path whereby the web reflects said light beam;and a second rod-shaped light-conducting member at least partiallyembedded in said support means to direct said reflected light beamagainst said photosensitive element, each of said light-conductingmembers having a' smooth end face adjacent to said portion of said pathand located in a plane which is substantially normal to the axis of therespective light-conducting member, the axes of said light-conductingmembers making identical acute angles with a plane which includes saidpoint and is located midway between said light-conducting members.

'2. Apparatus as defined in claim 1, wherein the ex-I tensions of axesof said light-conducting members intersect each other at said point ofsaid portion of said path. 3. Apparatus as defined in claim 1, whereinsaid sup port means is provided with chambers for said light source andsaid photosensitive element. v

4. Apparatus as defined in claim 1, wherein said means for biasingcomprises at least one roller adjacent to said portion of said pathopposite said guide surface.

5. Apparatus as defined in claim 1, wherein said guide surfaceisprovided witha continuous groove through which the light from saidsource passes between said point and said members.

6. Apparatus as defined in-claim 5, wherein the width of said grooveslightly exceeds the diameters of said members. I

7. Apparatus as defined in claim 1, further comprising at least oneadditional pair of light-conducting members in said support means, anadditional light source behind one of said additional members and anadditional photosensitive element behind the other of said additionalmembers, said additional members reference to said first-mentioned pointtransversely of said path.

8. Apparatus as defined in claim 7, wherein said support means defines afirst chamber behind said one members and a second chamber behind saidother members, and further comprising a first insert removablyaccommodating said light sources and a second insert removablyaccommodating said elements, said first and second inserts beingrespectively received in said first and second chambers.

1. Apparatus for photoelectrically scanning webs of flexible material,such as photographic films or paper webs, particularly to detectportions of reduced reflectivity, comprising support means having aconvex guide surface defining a portion of an elongated path for webs offlexible material; means for advancing successive webs along said pathin a predetermined direction; means for biasing the webs in said portionof said path against said guide surface; a light source removablymounted in said support means; a photosensitive element removablymounted in said support means; a first rod-shaped light-conductingmember at least partially embedded in said support means between saidlight source and said guide surface to direct a beam of light whichissues from said light source against the web at a predetermined pointof said portion of said path whereby the web reflects said light beam;and a second rod-shaped light-conducting member at least partiallyembedded in said support means to direct said reflected light beamagainst said photosensitive element, each of said light-conductingmembers having a smooth end face adjacent to said portion of said pathand located in a plane which is substantially normal to the axis of therespective lightconducting member, the axes of said light-conductingmembers making identical acute angles with a plane which includes saidpoint and is located midway between said light-conducting members. 2.Apparatus as defined in claim 1, wherein the extensions of axes of saidlight-conducting members intersect each other at said point of saidportion of said path.
 3. Apparatus as defined in claim 1, wherein saidsupport means is provided with chambers for said light source and saidphotosensitive element.
 4. Apparatus as defined in claim 1, wherein saidmeans for biasing compriSes at least one roller adjacent to said portionof said path opposite said guide surface.
 5. Apparatus as defined inclaim 1, wherein said guide surface is provided with a continuous groovethrough which the light from said source passes between said point andsaid members.
 6. Apparatus as defined in claim 5, wherein the width ofsaid groove slightly exceeds the diameters of said members.
 7. Apparatusas defined in claim 1, further comprising at least one additional pairof light-conducting members in said support means, an additional lightsource behind one of said additional members and an additionalphotosensitive element behind the other of said additional members, saidadditional members being laterally offset with reference to saidfirst-mentioned members so as to cause a light beam issuing from saidone additional member to impinge on a web in said path at a second pointwhich is offset with reference to said first-mentioned pointtransversely of said path.
 8. Apparatus as defined in claim 7, whereinsaid support means defines a first chamber behind said one members and asecond chamber behind said other members, and further comprising a firstinsert removably accommodating said light sources and a second insertremovably accommodating said elements, said first and second insertsbeing respectively received in said first and second chambers.